1. Field of the Invention
This invention relates to an improved fluid catalytic cracking process which comprises cracking heavy fraction oils to obtain therefrom light fraction oils such as gasoline and kerosene. More particularly, it relates to such a process which comprises catalytically cracking, in the presence of a particulate iron oxide (ferrite)-containing catalyst, heavy fraction oils including 0.5 ppm or more in total of at least nickel and vanadium among heavy metals such is as particularly nickel, vanadium, iron and copper, separating a portion of the particulate catalyst with the heavy metals deposited thereon in a high concentration (the catalyst portion being magnetically attachable catalyst particles) from an equilibrated particulate catalyst produced from said iron oxide-containing particulate catalyst during its use, by the use of a magnetic separator, and then recycling to the system another portion of the particulate catalyst with the heavy metals deposited thereon in a low concentration (the other catalyst portion being magnetically unattachable catalyst particles), together with a particulate ferrite-containing catalyst as a makeup or replenishment, so that it is possible to maintain the performance of the apparatus for carrying out said process at a high level.
2. Prior Art
In conventional catalytic cracking, petroleum derived hydrocarbons are contacted with a catalyst for cracking so as to obtain a large quantity of light oil fractions such as LPG and gasoline as well as a small quantity of a cracked light oil, and, further, coke deposited on the catalyst is burnt with air for removal thereof to recyle the thus treated catalyst for reuse. As starting oils in this case, there have heretofore been mainly used so-called distillates such as a light gas oil (LGO) and heavy gas oil (HGO) from an atmospheric-pressure distilling column and a vacuum gas oil (VGO) from a reduced-pressure distilling column.
However, due to the recent world-wide necessity of using heavier crude oils and a change in demand for petroleum products in our country, a tendency of overproduction of heavy oils and the like has been appreciated from the standpoint of both demand and supply of the petroleum products; and therefore, it has been necessary that heavy fraction oils including distillation residues be used as starting oils for use in a catalytic cracking process.
It is known, however that heavy fraction oils including distillation residues contain metals such as nickel, vanadium, iron, copper and sodium in a far greater total amount than distillates, and that these metals will be deposited on a catalyst so that they hinder the activity and selectivity of the catalyst when the catalyst is used in catalytic cracking. In other words, the cracking rate will gradually decrease as the metals accumulate on the catalyst so that it is substantially impossible to attain a desired cracking rate, while the amount of hydrogen evolved and the amount of coke produced will remarkably increase thereby making it difficult to operate equipment for carrying out the cracking. Further, at the same time, desired liquid products will be obtained in a decreased yield. Among said metals, particularly vanadium will destroy zeolite which is the active component of the catalyst so that the catalytic activity is lowered. Nickel has no action which decreases the catalytic activity as vanadium does, but it will remarkably increase hydrogen and carbon due to its dehydrogenating catalytic activity.
To relieve such effects of the contaminating metals on the catalyst in the system, there has usually been employed a process which comprises withdrawing periodically or continuously a portion of the particulate equilibrated catalyst present in the system and, instead, replenishing a necessary amount of a fresh particulate catalyst therein, whereby the activity of the equilibrium catalyst is maintained. In this case, it is necessary that the particulate catalyst be withdrawn in a remarkably large amount, this being very economically disadvantageous and raising a serious problem particularly in case of the fluid catalytic cracking of a residual oil containing metals in a large amount.
As measures for solving this problem, a method for removing metals deposited on catalysts and a method for inhibiting the activity of the metals are known. For example, as the above removing method, there has been proposed a method for chemically treating the withdrawn equilibrium catalyst to remove the heavy metals therefrom for reuse of the thus treated catalyst (F. J. Elvin et al, NPRA Annual Meeting, AM-86-41). The method so proposed will inevitably discharge a large amount of waste liquid which requires substantial expenses from the standpoint of preventing environmental pollution.
As the above inhibiting method, a method which comprises adding a metal scavenger to the catalyst and a method which comprises adding to a starting oil a metal passivator such as antimony (U.S. Pat. Nos. 3,711,422 and 4,025,458) or bismuth (U.S. Pat. Nos. 4,083,807 and 3,977,963)are known. In addition, it is known that alkaline earth metal compounds are effective as the metal passivators (for example, Japanese Pat. Appln. Laid-Open Gazettes Nos. Sho 61-204041, Sho 60-71041, Sho 61-278351 and Sho 63-123804).
Even in these methods, it is not possible yet to fully prevent the contaminating metals from exerting their effects. Accordingly, in order to maintain the activity of the catalyst, it is necessary to withdraw the equilibrated catalyst partly from the system and, instead, a necessary amount of a fresh catalyst has to be replenished. When said catalyst exchange is effected, a portion of the equilibrium catalyst particles to be withdrawn contain those having still high catalytic activity. Thus, it follows that said catalyst exchange method uses the catalyst inefficiently.
The present inventors of this application have already found that a portion of the particulate equilibrated catalyst on which the heavy metals are deposited is withdrawn from the system, the catalyst so withdrawn is separated by the use of a highly gradient magnetic separator into one catalyst portion on which more metals are deposited and the other one on which less metals are deposited and the less metals-deposited catalyst portion is then recycled to the system, whereby the activity of the equilibrated catalyst is enhanced and the selectivity thereof is remarkably improved (Japanese Patent Gazettes Nos. 63-37156 and 3-37835). This technique disclosed in said Gazettes never conflicts with anti-metal measures such as the above-mentioned chemical treatment, metal scavengers and metal passivators and can be used together with them. In such a method which comprises separating the equilibrated catalyst by the use of a magnetic separator into a more metal deposited portion and a less metal deposited portion, it is important how to effect such separation precisely depending on the concentrations of metals deposited on the particulate catalyst, and the separation can be achieved more effectively as the difference in magnetizabillty (magnetic susceptibility) is greater between the more metal deposited catalyst particles and the less metal deposited ones.